The present invention relates to a method for forming polyesters from terephthalic acid and ethylene glycol and, more particularly, to a method for forming polyesters (polyethylene terephthalate) which possess improved coloration and low diethylene glycol (DEG) content.
In recent years, polyesters, especially polyethylene terephthalate (PET), have been used in manufacturing fibers for clothes, film for magnetic tapes, food containers and resins. In order to be used in products such as these, polyethylene terephthalate needs to possess improved coloration, physical properties and a high degree of polymerization.
There are several conventional methods for preparing polyethylene terephthalate. For example, in the method of direct esterification, terephthalic acid and ethylene glycol are heated under either atmospheric conditions or at elevated pressure at temperatures ranging from about 200.degree. C. to about 280.degree. C. Another method, referred to as ester interchange, comprises heating dimethyl terephthalate and ethylene glycol in the presence of catalysts at temperatures ranging from about 160.degree. C. to 240.degree. C. Subsequently, polymers having a high degree of polymerization (exceeding 100) are prepared by continuously polycondensating the bis(beta-hydroxyethyl) terephthalate and/or low polymers thereof (oligomers) obtained from the esterification step in the presence of polycondensation catalysts in a high vacuum (not exceeding 1.0 torr) at temperatures ranging from 260.degree. C. to 300.degree. C. The above-mentioned direct esterification process is preferable to the ester interchange process because of its relatively low cost.
In a reaction for forming polyesters, reaction catalysts, for example, compounds of metals such as antimony, titanium, germanium, tin, zinc, manganese and lead, are generally used to accelerate the reaction. It is also well known that the color and thermal stability of the resulting polyester products and the reaction kinetics are notably changed depending on the reaction catalysts used. Typically, the reaction for forming polyesters is conducted over a long period of time and at a high temperature in the presence of metallic catalysts. This is because the reactions used for forming polyesters having a high degree of polymerization in a short period of time are accompanied by several undesirable side reactions that result in the polyester products being yellow in color, having an increased content of DEG, and a concentration of terminal carboxyl groups in excess of optimum levels. Consequently, the physical properties of the polyesters, for example, their melting point, strength, and the like, are deteriorated.
There are several methods for solving the above-mentioned problems, one of which is by using newly developed catalysts. However, there is no known catalyst which can be substituted for the compounds of antimony. These antimony compounds, especially antimony trioxide, are used because they are inexpensive and have good catalytic properties and thermal stability.
The catalysts being used in the above-mentioned conventional methods for forming polyesters, however, have a negative influence on the physical properties of the resulting polyester products. That is, almost of the catalysts used in the conventional methods discolor the resulting polyester products (for example, U.S. Pat. No. 3,927,052, Japanese Laid-open Patent Publication No. 1-245,015) or increase the content of diethylene glycols (Japanese Patent Publication No. 62-265,324) thereby causing a deteriation of the physical properties of the polyesters.